Methods and systems for determining the average atomic number and mass of materials

ABSTRACT

Disclosed herein are methods and systems of scanning a target for potential threats using the energy spectra of photons scattered from the target to determine the spatial distributions of average atomic number and/or mass in the target. An exemplary method comprises: illuminating each of a plurality of voxels of the target with a photon beam; determining an incident flux upon each voxel; measuring the energy spectrum of photons scattered from the voxel; determining, using the energy spectrum, the average atomic number in the voxel; and determining the mass in the voxel using the incident flux, the average atomic number of the material in the voxel, the energy spectrum, and a scattering kernel corresponding to the voxel. An exemplary system may use threat detection heuristics to determine whether to trigger further action based upon the average atomic number and/or mass of the voxels.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 13/275,909 filed onOct. 18, 2011, U.S. Ser. No. 12/578,956 filed on Oct. 14, 2009, U.S.Ser. No. 11/854,213 filed on Sep. 12, 2007, and U.S. Ser. No. 11/177,758filed on Jul. 8, 2005, which claims priority to U.S. Ser. No. 60/586,351filed on Jul. 8, 2004 and is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of non-intrusive scanning, and moreparticularly to systems and methods of determining the average atomicnumber and mass of a target or one or more portions of a target, as wellas to systems and methods of obtaining limits on the mass of certainelements present in a target or in one or more portions of a target.

2. Background Information

A desirable characteristic of a non-intrusive inspection technique forinspecting a target is the ability to rapidly determine atomic number(Z) and density of the target material, as well as the spatialdistribution of atomic number and density. In particular, a rapiddetermination, preferably with low radiation dose, of the distributionin three dimensions of average atomic number and/or mass is a powerfuland useful means to determine the contents of a target container. Thisinformation may be used to determine a probability that a targetcontainer such as a piece of luggage, a shipping container, a storagecontainer, or other container for land, sea, or air transport contains acertain material, such as for example a high-Z and/or a high-densitymaterial. Knowledge that a target container contains such a material maybe used to identify a threat. For example, the presence of lead in acontainer may indicate shielding for a “dirty bomb” or other radioactivematerial. The presence of high Z materials such as uranium may signalthe presence of a nuclear weapon in the container. Moreover, measurementof mass distribution, average atomic number, or both may form part of asequence of inspection techniques. For example, regions of the targetthat are determined to match specified Z and density categories can beused as input into other inspection techniques that will further probethese regions. Such a system may offer the advantage of providing threatdeterminations in a convenient time scale.

Techniques for the non-intrusive inspection of cargo include thedetection of transmitted radiation (as in x-ray imaging) to obtain atwo-dimensional representation of the distribution of density in atarget cargo container. Two-dimensional imaging using scatteredradiation, such as Compton-scattered radiation, has also beendemonstrated. Because of the limitations of two-dimensional imaging, itis often desirable to obtain the density distribution in threedimensions. Non-intrusive scanning to obtain both two- andthree-dimensional images of a target using nuclear resonancefluorescence techniques has been described, for example, in U.S. Pat.No. 5,115,459, Explosives Detection Using Resonance Fluorescence ofBremsstrahlung Radiation, and U.S. Pat. No. 5,420,905, Detection ofExplosives and Other Materials Using Resonance Fluorescence, ResonanceAbsorption, and Other Electromagnetic Processes with BremsstrahlungRadiation, the contents of both of which are hereby incorporated byreference.

SUMMARY OF THE INVENTION

Methods and systems for achieving non-intrusive inspection of cargo,shipping containers, luggage, and other targets via measurements ofscattered photon energy spectra resulting from the scattering of photonbeams are presented here. The methods and systems presented here takeadvantage of processes that contribute to the 511 keV annihilation peakand higher-order processes, some occurring at higher energies, whichexhibit stronger Z-dependence than such processes as Compton orRutherford scattering. The methods and systems disclosed also benefitfrom the greater penetration of radiation that occurs at energies abovethe K-edge for most materials, allowing fewer losses in the target ofphotons that would otherwise contribute to signal from deep in thetarget. This allows the inspection of larger and/or denser targets thanmay be practicable with systems operating at lower photon energy.Additionally, the methods presented here may be employed to obtain atwo-dimensional and three-dimensional average atomic number and/ordensity as well as mass limits on the presence of high-Z material, datamore rapidly and with lower radiation dose than NRF imaging.

In one aspect, a method for analyzing the material in a voxel of atarget comprises illuminating the voxel with a photon beam; measuring afirst number of photons scattered from the voxel in a first energy rangeand in a first measurement direction; measuring a second number ofphotons scattered from the voxel in a second energy range and in asecond measurement direction; determining a ratio of the first number ofphotons to the second number of photons; and determining an averageatomic number of the material in the voxel using the ratio. In furtherembodiments, the first energy range includes 511 keV. In furtherembodiments, the second energy range excludes 511 keV. In otherembodiments, the first direction is the same as the second direction.

In another aspect, a method for analyzing the material in a voxel of atarget, comprises illuminating the voxel with a photon beam; measuringan energy spectrum of photons scattered from the voxel in a measurementdirection; determining a first number of photons contributing to theenergy spectrum in a first energy range; determining a second number ofphotons contributing to the energy spectrum in a second energy range;computing a ratio of the first number of photons to the second number ofphotons; and determining an average atomic number of the material in thevoxel using the ratio. In further embodiments, the first energy rangeincludes 511 keV. In further embodiments, the second energy rangeexcludes 511 keV.

In another aspect, a system for analyzing the material in a voxel of atarget comprises a device for generating a photon beam; a first detectorconfigured to detect a first energy spectrum of photons scattered fromthe voxel in a first measurement direction; and a processor; wherein theprocessor is configured to determine a ratio of a first number ofscattered photons having energies in a first energy range to a secondnumber of scattered photons having energies in a second energy range;and wherein the processor is further configured to determine the averageatomic number in the voxel. In other embodiments, a system for analyzingthe material in a voxel of a target further comprises a second detectorconfigured to detect a second energy spectrum of photons scattered fromthe voxel in a second measurement direction. In still furtherembodiments, the first angle is the same as the second angle. In stillfurther embodiments, the first energy range includes 511 keV. In stillfurther embodiments, the second energy range excludes 511 keV.

In another aspect, a method of analyzing material in a voxel of a targetcomprises illuminating the voxel with a photon beam; determining anincident flux upon the voxel; measuring at least one energy spectrum ofphotons scattered from the voxel; determining, using the measured energyspectrum, the average atomic number in the voxel; and determining themass in the voxel using the incident flux, the average atomic number ofthe material in the voxel, the measured energy spectrum, andpredetermined values of a scattering kernel corresponding to the voxel.In still further embodiments, determining the average atomic number inthe voxel comprises determining a first number of photons contributingto at least one of the energy spectra in a first energy range;determining a second number of photons contributing to at least one ofthe energy spectra in a second energy range; computing a ratio of thefirst number of photons to the second number of photons; and determiningan average atomic number of the material in the voxel using the ratio.In still further embodiments, the first energy range includes 511 keV.In still further embodiments, the second energy range excludes 511 keV.

In another aspect, method of analyzing material in a plurality of voxelsof a target, comprises (a) illuminating the voxels with a photon beam;(b) measuring, for each of the voxels, at least one energy spectrum ofphotons scattered from the voxel; (c) for each of the voxels,determining the average atomic number of the material in the voxel,using the measured energy spectrum; and (d) for each voxel, (i)determining an incident flux upon the voxel; and (ii) determining themass in the voxel using the incident flux, the average atomic number ofthe material in the voxel, the measured energy spectrum, andpredetermined values of a scattering kernel corresponding to the voxel.In further embodiments, determining the average atomic number in each ofthe voxels comprises determining a first number of photons contributingto at least one of the energy spectra in a first energy range;determining a second number of photons contributing to at least one ofthe energy spectra in a second energy range; computing a ratio of thefirst number of photons to the second number of photons; and determiningan average atomic number of the material in the voxel using the ratio.In further embodiments, the first energy range includes 511 keV. Instill further embodiments, the second energy range excludes 511 keV.

In another aspect, a method of analysing material in a plurality ofvoxels of a target comprises (a) illuminating the voxels with a photonbeam; (b) measuring at least one energy spectrum of photons scatteredfrom each of the voxels; (c) determining, using the measured energyspectrum, the average atomic number in each of the voxels; (d) for eachvoxel, (i) determining a flux of photons incident on the voxel; and (ii)using the average atomic number, the measured energy spectrum, andpredetermined values of a scattering kernel to determine the averagemass in the voxel; (e) computing, using the estimated average mass andaverage atomic number in each voxel, an estimated exit flux exiting thetarget; measuring a measured exit flux exiting the target; computing adifference between the estimated exit flux and the measured exit flux;and computing a correction to the estimated average mass in each voxelbased upon the computed difference between the estimated exit flux andthe exit flux. In further embodiments, computing a correction to theestimated average mass in each voxel further comprises assigning, foreach voxel, a contribution to the computed difference between theestimated exit flux and the exit flux in proportion to the estimatedaverage mass in that voxel. In further embodiments, computing acorrection to the estimated average mass in each voxel further comprisesusing a minimization procedure to adjust the estimated average mass ineach voxel so that the computed difference between the estimated exitflux and the exit flux is minimized. In still further embodiments,computing a correction to the estimated average mass in each voxelfurther comprises adjusting the computed average atomic number in eachvoxel so that the computed difference between the estimated exit fluxand the exit flux is minimized.

In another aspect, a method of analyzing material in a voxel of a targetcomprises (a) illuminating the voxel with a photon beam; (b) determiningan incident flux incident on the voxel; (c) measuring at least oneenergy spectrum of photons scattered from the voxel; (d) determining themass in the voxel by (i) determining, using the incident flux and themeasured energy spectrum, the average atomic number in the voxel; and(ii) using the average atomic number, the measured energy spectrum, andpredetermined values of a scattering kernel to determine the mass in thevoxel; and (e) determining an upper limit on a mass of a selectedspecies present in the voxel, using the measured energy spectrum and themass in the voxel. In further embodiments, determining an upper limit ona mass of selected species present in the voxel comprises evaluating afirst scattering kernel corresponding to the selected species, andevaluating a second scattering kernel corresponding to a second species.In still further embodiments, determining the average atomic number ineach voxel comprises determining a first number of photons contributingto at least one of the energy spectra in a first energy range;determining a second number of photons contributing to at least one ofthe energy spectra in a second energy range; computing a ratio of thefirst number of photons to the second number of photons; and determiningan average atomic number of the material in the voxel using the ratio.In still further embodiments, the first energy range includes 511 keV.In still further embodiments, the second energy range excludes 511 keV.

In another aspect, a system for determining the average atomic number ina voxel of a target, the system comprises a photon beam; a means fordetermining an incident flux incident on the voxel; a detectorconfigured to view the target and equipped to detect an energy spectrumof photons scattered from the voxel; and a processor; wherein theprocessor is configured to determine, using the energy spectrum, theaverage atomic number in the voxel; and the processor is furtherconfigured to determine the average atomic mass in the target voxelusing the incident flux, the average atomic number, the energy spectrum,and predetermined values of a scattering kernel.

In another aspect, a method of analyzing material in a voxel of atarget, comprises illuminating the voxel with a photon beam; determiningan incident flux upon the voxel; measuring at least one energy spectrumof photons scattered from the voxel; determining a first number ofphotons contributing to at least one of the energy spectra in a firstenergy range, the first energy range including 511 keV; determining asecond number of photons contributing to at least one of the energyspectra in a second energy range; computing a ratio of the first numberof photons to the second number of photons; using a correlation betweenthe ratio and the first number of photons to determine a probableaverage atomic number and mass in the voxel. In further embodiments, thesecond energy range excludes 511 keV.

The present disclosure also describes methods and systems for scanning atarget for potential threats. In one aspect, a method of scanning atarget for potential threats comprises: (a) for each of a plurality ofvoxels in the target: (i) illuminating the voxel with a photon beam;(ii) measuring an energy spectrum of photons scattered from the voxel ina measurement direction; (iii) determining a first number of photonscontributing to the energy spectrum in a first energy range; (iv)determining a second number of photons contributing to the energyspectrum in a second energy range; (v) computing a ratio of the firstnumber of photons to the second number of photons; and (vi) determiningan average atomic number of the material in the voxel using the ratio;and (b) determining whether to trigger further action using the averageatomic numbers of each of the plurality of voxels. In furtherembodiments, the first energy range includes 511 keV. In still furtherembodiments the second energy range excludes 511 keV. In still furtherembodiments, the method further comprises displaying the spatialdistribution of the average atomic numbers on an output device. In stillfurther embodiments, further action comprises scanning a portion of thetarget by nuclear resonance fluorescence, or notifying an operator thatsuspicious material may be present.

In another aspect, a method of scanning a target for potential threats,comprises (a) for each of a plurality of voxels in the target: (i)illuminating the voxel with a photon beam; (ii) measuring a first numberof photons scattered from the voxel in a first energy range and in afirst measurement direction; (iii) measuring a second number of photonsscattered from the voxel in a second energy range and in a secondmeasurement direction; (iv) determining a ratio of the first number ofphotons to the second number of photons; and (v) determining an averageatomic number of the material in the voxel using the ratio; and (b)determining whether to trigger further action using the average atomicnumbers of the material in each of the plurality of voxels. In furtherembodiments, the first energy range includes 511 keV. In still furtherembodiments, the second energy range excludes 511 keV. In still furtherembodiments, the method further comprises displaying the spatialdistribution of the average atomic numbers on an output device. In stillfurther embodiments, further action comprises scanning a portion of thetarget by nuclear resonance fluorescence, or notifying an operator thatsuspicious material may be present.

In another aspect, a method of scanning a target for potential threats,comprises (a) for each of a plurality of voxels in the target; (i)illuminating the voxel with a photon beam; (ii) determining an incidentflux upon the voxel; (iii) measuring at least one energy spectrum ofphotons scattered from the voxel; (iv) determining, using the measuredenergy spectrum, the average atomic number in the voxel; and (v)determining the mass in the voxel using the incident flux, the averageatomic number of the material in the voxel, the measured energyspectrum, and predetermined values of a scattering kernel correspondingto the voxel; determining whether to trigger further action using themasses and the average atomic numbers in each of the plurality ofvoxels.

In another aspect, a system for scanning a target for threateningmaterial comprising: a means for generating a beam of photons; a meansfor translating the target relative to the beam of photons; at least onedetector configured to detect at least one energy spectrum of photonsscattered in a measurement direction from at least one voxel of thetarget; and a processor; wherein the processor is configured todetermine a ratio of a first number of scattered photons having energiesin a first energy range to a second number of scattered photons havingenergies in a second energy range; the processor is further configuredto determine the average atomic number in the voxel; the processor isfurther configured to use the average atomic number in the voxel todetermine whether to trigger further action.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a schematic diagram of an exemplary embodiment of a scannerconfiguration;

FIG. 2 is a schematic diagram of a detail of an exemplary apparatus fordetermining average atomic number and mass in several voxels of a targetcontainer;

FIG. 3 is a plot of the energy distribution of scattered high-energyphotons from target samples of lead, copper, and boron oxide (B₂O₃)surrounded by paper;

FIG. 4 illustrates the atomic number of target sample as a function ofratio of scattered photon intensities at 511 keV and at 600 keV; and

FIG. 5 is a plot of the ratio R(Z) against the number of counts in the511 keV annihilation peak, for samples of a variety of masses andaverage atomic numbers.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

To provide an overall understanding, certain illustrative embodimentswill now be described; however, it will be understood by one of ordinaryskill in the art that the devices and methods described herein can beadapted and modified to provide devices and methods for other suitableapplications and that other additions and modifications can be madewithout departing from the scope of the systems described herein.

Unless otherwise specified, the illustrated embodiments can beunderstood as providing exemplary features of varying detail of certainembodiments, and therefore, unless otherwise specified, features,components, modules, and/or aspects of the illustrations can beotherwise combined, specified, interchanged, and/or rearranged withoutdeparting from the disclosed devices or methods. Additionally, theshapes and sizes of components are also exemplary, and unless otherwisespecified, can be altered without affecting the disclosed devices ormethods.

Some exemplary systems for employing continuous-spectrum, photon sourcessuch as bremsstrahlung sources in resonant scattering measurements (alsocalled nuclear resonance fluorescence or NRF) in non-intrusive scanningapplications are discussed in U.S. Pat. Nos. 5,115,459, and 5,420,905.The methods and systems described herein may employ similar apparatusfor measurements using non-resonant scattering processes.

A schematic diagram of an exemplary embodiment of a scannerconfiguration is shown in FIG. 1.

The system includes a photon source 12 producing photons having anenergy spectrum over some energy range. Suitable photon sources mayinclude: a bremsstrahlung source; a Compton-broadened photon sourceusing nuclear decay from a radioactive source; coherent bremsstrahlungradiation; free electron lasers; laser backscatter from high energyelectrons; neutron capture photons; or other photon sources known tothose skilled in the field.

In the embodiment illustrated in FIG. 1, the photon source 12 may be abremsstrahlung source and may include an electron source 14 providing abeam of electrons 32 incident on a bremsstrahlung target 16 to generatea bremsstrahlung photon beam 34. The bremsstrahlung target 16 may befollowed by a beam stopper (not illustrated) to stop the electrons 32. Afilter 52 may follow the beam stopper to filter out low energy photonsfrom the bremsstrahlung beam 34 if desired. A collimator 18 may beemployed to collimate the bremsstrahlung beam 34. Shielding (notillustrated) may enclose the photon source 12. A description of anexemplary suitable bremsstrahlung photon source may be found in U.S.Pat. No. 5,115,459.

A target 20 to be scanned, such as a cargo container, shippingcontainer, luggage, package, or other container or object, may be placedin the path of the photon beam 34. In one embodiment, the target may bemoved through the path of the beam, for example by a conveyor belt. Inanother embodiment, the beam 34 may be scanned across the target 20, forexample, by moving photon source 12 or steering the electron beam 32.Other ways of achieving scanning of the photon beam 34 over the targetcontainer 20 will be recognized by those skilled in the art. The target20 may contain target contents 22. The incident photon beam 34 impingesupon the target container 20, and photons 48 may be both scattered fromthe contents 22 and the target 20 as well as transmitted through thecontents 22 and the target 20.

Detecting apparatuses 38 and 40, which may include an array of detectors42, may capture, measure, count, and/or record the energies of thephotons scattered in a given direction or directions. A description ofseveral exemplary suitable detecting apparatuses may be found in U.S.Pat. No. 5,115,459. The detecting apparatus 38 or 40 may further includea filter over the face of each detector to absorb low energy photons,and shielding (not illustrated). As scattering from the collimatingaperture 18 could lead to a significant amount of photons directedtoward the detecting apparatus 38 or 40, a shadow shield (notillustrated) between the collimator and the detecting apparatus 38 or 40may be employed. A beam dump 30 may be provided to absorb the energy ofthe beam 34 that is not absorbed as the beam 34 passes through thetarget 20. Shielding (not shown) may enclose the entire device whileallowing convenient means for the entry and exit of targets.

Data from the detecting apparatus 38 or 40 is sent to a processor 46which may analyze the data, for example, to determine an average atomicnumber, mass, or upper limits on a mass of a selected element asdescribed below. The data may be preprocessed by preprocessingelectronics 44, which may include preamplifiers, filters, timingelectronics, and/or other appropriate preprocessing electronics.(Although the preprocessing electronics 44 are connected serially in thefigure, they may also be connected in parallel to the processor 46.) Theprocessor 46 may be further adapted to evaluate the data to determinewhether the contents of the target volume meet or exceed one or morepredetermined detection thresholds. For example, the processor 46 maycompare the data for each irradiated target volume to profiles of“normal” target volumes to determine whether the irradiated targetvolume should be considered “suspicious.” In addition, the processor 46may be programmed with other threat detection heuristics as describedbelow. Further, as described in more detail below, the processor 46 maycontrol a variety of parameters of the photon beam, scanning, detection,and/or other aspects of the system.

The detecting apparatus 38 and/or 40 may be configured to view thetarget 20 at a measurement angle θ with respect to the photon beam 34 ofmore than 90 degrees relative to the direction of the photon beam. In anexemplary embodiment, the detecting apparatus 40 views the target at ameasurement angle of approximately 130 degrees.

The beam 34 passes through the target contents 22. This beam may beabsorbed in a beam dump 30 designed to absorb substantially all of theremaining energy. For example, a suitable beam dump for 10 MeV mayinclude a layer of a hydrogenous material containing boron or lithium, alayer of carbon, and a layer of iron in a cavity formed in a shield oflead and/or iron to shield the sides and the detectors fromback-streaming low energy photons. A layer of a hydrogenous materialcontaining boron or lithium may surround the outside of this shield. Thedepth of this cavity, the beam dimensions, the directional collimationof the detectors, and the exact location of the detectors are relatedparameters that may be made compatible so as to minimize the number ofbackward-streaming photons from the beam dump entering the detectors.Additional shadow shields may be set up to help meet this goal.

Scanning can be achieved in a variety of ways with the techniquedescribed herein. The luggage can be scanned with the beam by moving theentire photon source 12, the target 20, or simply the aperture 18. Theelectron beam may also be deflected by a magnet to sweep the photon beamdirection. Preferred photon beam geometries include spots (cones) andstripes. Other suitable scanning configurations, geometrics, andpatterns may be recognized by those skilled in the art and may beemployed.

For example, in one embodiment, if the beam 34 is collimated using asmall circular aperture 18 to an average angle of approximately 1/20radians (about 3 degrees), the spot 1 meter from the aperture may beabout 10 cm across, a suitable size for imaging the contents of a pieceof luggage. If higher resolution is desired, a more tightly collimatedbeam may be used. Alternatively, larger spot sizes may be preferred forrapid scanning and/or for larger targets.

If the photon beam 34 is collimated using a vertical slit aperture toproduce a thin stripe of 10 cm width at the point of incidence with apiece of luggage, for example, a 60 cm long suitcase could be scanned ina few seconds as the suitcase moves on a conveyor belt. Alternatively,the photon beam 34 could be collimated into a spot swept vertically byan adjustable collimator or by magnetic deflection of the electron beam32 used to generate the photon beam 34. Even if the collimation is inthe form of a vertical stripe, the central intensity remains thehighest, reflecting the natural collimation, and magnetic deflection ofthe electron beam 32 may be useful for imaging. In other embodiments,stripes with configurations other than vertical may be employed.

In addition, in some embodiments, a pulsed photon beam may be used toallow spatial resolution. Where the photon source is generated by anelectron accelerator with suitable pulse structure, the relative timebetween the photon pulse and the time of detection can be used to inferthe spatial position of possible interaction points.

A target 20 may be conceptualized as divided into portions or “voxels.”Each voxel is defined by the three dimensional intersection of thephoton beam 34 with the view of a collimated detector 42. One such voxel50 is identified in FIG. 1.

The processor 46 may be adapted to analyze the data obtained by thedetecting apparatus in any combination of 38 and/or 40. For example, theprocessor 46 may be programmed with statistical information about massor atomic number distributions that fall within “normal” profiles. Atarget volume or voxel 50 or a combination of volumes or voxels 50 whichdeviates significantly from these profiles may be identified as“suspicious.” The processor 46 can also be adapted to compare data tostored profiles indicative of a high likelihood of a particular type ofthreat. For example, if a region of a target shows the explicitelemental profile of an explosive substance, or of high-densitymaterials typically used as radioactive shielding, the processor 46 maysignal a positive threat detection event. The system may respond to apositive threat detection event in any of a variety of ways, includingrescanning the region at a higher resolution, performing a differenttype of scan (such as a NRF scan) for particular materials, displayingan image of the target contents, and/or signaling a human operator.

The detection methods thus described, in which scattering from thetarget 20 and target contents 22 is detected by detectors 42 in detectorarray 38 and/or 40, may be employed to obtain three-dimensional imagingof the target contents 22. For example, if the average atomic number isdetermined in each voxel 50 according to the methods described herein,then these data may, if desired, be reconstructed as a three-dimensionalimage of the target contents 22 displaying the spatial distribution ofaverage atomic number. Similarly, using measurements of the mass in eachvoxel, a three-dimensional distribution of mass throughout the targetmay be constructed as an image.

In the embodiment illustrated in FIG. 1, the system may also include adirect transmission detector 24, such as an X-ray imager, which canmeasure the intensity and/or energy of photons transmitted through thetarget 20 as a function of the position at which the photon beam 34strikes the target 20 (or, for a bremsstrahlung source, as a function ofthe position at which the electron beam strikes the bremsstrahlungtarget). Such a measurement could be used, for example, to obtain a mapof the average density of the target 20, projected along the axis of thephoton beam 34. In this way, a very precise image of the transmissiondensity of the target can be constructed. Such an image will identifyspecific areas of high material density in the target which would be afurther aid in detecting explosive or high atomic number materials.(Similar density imaging could also be achieved by detecting theback-scatter from the target 20, especially at low energies). Atransmission detector 24 may also be employed to measure the totalphoton flux transmitted through the target 20, with or without spatialresolution, as a function of energy.

An exemplary system for achieving non-intrusive inspection of a targetcontainer is further illustrated schematically in FIG. 2. A collimatedbeam 34 of photons impinges on the target 20. This collimated photonbeam 34, as described above, may have a continuous spectrum in an energyregion of interest, and can be produced by a variety of mechanisms suchas bremsstrahlung, radioactive decay, or other means known in the art.The intensity and energy of the photon beam before impinging on thetarget (the “incident flux” F₁) is known, either from design and controlparameters, by direct measurement, or both.

In one embodiment, the incident flux F₁ on the target 20 may be measuredand/or monitored by inserting a flux measurement device at position 72in the photon beam. The flux measurement device may be, for example, anelectrometer and/or an ionization gauge, where the photon beam passesthrough a material having known interaction cross-section with photonsat the energies of the photon beam 34. The flux measurement device thenmeasures the current created as the photon beam interacts with thematerial and outputs the incident flux F₁, preferably as a function ofphoton energy. (A similar flux measurement device 74 may be presentdownstream of the target 20 for measuring the exit flux, as discussedbelow).

Photons scattered from the target and/or its contents may be detected ina photon detector 42 or an array 38 of photon detectors, which may becollimated to view a particular portion of the target. The spatialvolume viewed by a photon detector in this example is determined by thecross sectional area of the beam and the detector view along thecollimation axis of the detector. (As noted above, the portion of thetarget volume viewed by a photon detector, the intersection in space ofthe path of the photon beam 34 with the field of view of a detector 42,is referred to as a “voxel,” which is a volume element of thethree-dimensional space of the target container. The precise shape ofeach voxel, which may be irregular, is determined by the shape andgeometry of the beam and the geometry of the collimated view of eachdetector.) In the embodiment illustrated schematically in FIG. 2, anarray 38 of detectors 42 may simultaneously view photons scattered frommultiple voxels, for example, voxels 62, 64, 66 along the beam 34. Thearray 38 of collimated photon detectors can be replaced by a singlephoton detector (or any number) that can change viewing volume withoutany loss of generality to the method herein described. The photondetectors 42 measure the number and energy distribution of photonsscattered in the measurement direction θ measured with respect to theincident beam direction. The rate (or integral or number of photons forfixed time counting) and shape of the energy spectrum of photonsscattered into each detector is dependent on (i) the intensity andenergy distribution of the incident photon flux reaching the voxelviewed by the detector; (ii) the density and composition of the viewedvoxel; (iii) the angle θ at which the detector is aimed, relative to theincident photon beam; (iv) the efficiency and energy resolution of thephoton detectors; and (v) the density and/or composition of the materialbetween the voxel and the detector.

FIG. 3 illustrates the dependence of the scattered photon energy spectraupon the composition of the portion of the target being probed. In FIG.3, the energy spectra of photons scattered at 130 degrees (relative tothe direction of the photon beam 34) from targets of lead, copper, andB₂O₃ powder surrounded by layers of paper sheets (magazines), are shownfor similar incident photon beams. An important feature in FIG. 3 is thelarge difference in amplitude of the photon energy spectra for energiesoutside the positron annihilation peak at 511 keV. As illustrated in thefigure, the photon intensity outside the annihilation peak increaseswith increasing Z. For example, as illustrated in the figure, the photonenergy spectrum for lead (Z=82) for energies greater than 511 keV showsapproximately an order of magnitude greater photon intensity over thatof other, lower-Z scattering materials of comparable or greaterirradiated mass. The photon energy spectrum for energies below the 511keV peak shows a similar increase in photon intensity with increased Z,and may also be used for the Z measurement technique described below.However, at energies below approximately 150 keV, the photon energyspectrum becomes dominated by the effects of the K edge in the targetmaterial and/or any shielding materials that may be present. These lowerenergies are therefore not ideal for the average Z measurement.

The features illustrated in FIG. 3 are general and may be observed inscattered photon energy spectra obtained by irradiation of a samplewith, for example, a photon bremsstrahlung beam with end-point energygreater than 1 MeV. As will be understood by those skilled in the art,similar photon energy spectra would be observable for a variety ofdetector geometries, including without limitation any detector placedwith a viewing angle of greater than approximately 90° relative to theincident photon beam.

The scattered photon energy spectra such as those illustrated in FIG. 3have a general and important feature that forms the basis of a rapiddetection scheme for identifying high-Z materials: The part of thephoton energy spectrum scattered at angles greater than approximately 90degrees relative to the bremsstrahlung beam and at energies well belowthe endpoint of the bremsstrahlung beam is very dependent on the Z ofthe material. As an example, a sample of lead presents about an order ofmagnitude more intensity than a similar mass of copper. The Z-dependenceof features of this photon energy spectrum can be used as describedbelow for identifying and/or mapping the average atomic number of thecontents of a target container.

In addition, as will be fully described below, the measured average Z ina voxel of the inspected target material, obtained by the methoddescribed below, together with the measured absolute scattered photonintensity can be used to determine the total mass of the voxel.

Exemplary Methods

The following methods may be used to non-intrusively inspect a targetcontainer and identify the average Z of the contents. As illustratedschematically in FIGS. 1 and 2, a photon beam 34 is made to impinge on atarget 20. The photon beam may be scanned perpendicular to a face of thetarget container. The target container may be moved through the systemperpendicular to the beam direction and to the scanning directionallowing every region of the container to be examined. As shownschematically in FIG. 2, for every position of the beam the collimateddetectors interrogate a voxel where the beam and the collimated view ofthe detectors intersect (the voxels interrogated by the detectors may bedisposed adjacently; the illustrated voxels 62, 64, and 66 of FIG. 2have been spatially separated for clarity of illustration). Theinspection system may include processor 46, programmed to dynamicallyanalyze the data from the detector array to rapidly determine theaverage atomic number Z, mass of material, and/or limits on the mass ofa particular material in each voxel irradiated by the beam, using themethods described below.

An exemplary inspection system may include data acquisition electronicsfor collecting photon counts and/or energy distribution from the photondetectors. In addition, the processor may be further programmed withthreat detection heuristics that determine whether to take furtheraction dependent upon the result of analyzing the collected data. Suchfurther action may include notifying a human operator of a potentialthreat or triggering further measurements to be initiated and/orconducted either by an automated system or by human operators. Forexample, the system may be programmed to alert an operator or conductfurther measurements upon detection of the presence of materials havingaverage Z in a specified range or above a threshold value. Such furthermeasurements may include higher-resolution scans of selected regions ofthe target. Further measurements may also include scanning the targetwith other scanning methods, including NRF imaging, for further analysisof the isotopic contents of the target or a portion of the target. Anexemplary system for determining the average Z and/or mass distributionin a target may use the same detectors for both the average Z and/ormass measurements, and any additional NRF imaging of the target.Alternatively, an additional set of detectors may be supplied forfurther imaging. As illustrated in FIG. 1, the system may includedetector arrays 38 and/or 40 that may be used for NRF imaging such asdescribed in U.S. Pat. Nos. 5,115,459, and 5,420,905. Also asillustrated in FIG. 1, the system may be configured for transmission NRFdetection using reference scatterers 28 and detector array 36, also asdescribed in U.S. Pat. Nos. 5,115,459, and 5,420,905.

In some embodiments, processor 46 may be configured to represent theaverage Z values and/or average density for each voxel graphically on avisual display, such as with color or shading, to form a two- orthree-dimensional image of the container's contents. The processor mayalso be configured to display regions of the container having averageatomic number above some threshold value.

Determination of the Average Atomic Number:

The average Z, S_(av.) of the material in a voxel is determined in thefirst approximation by the ratio of measured photon counts in two energyregions of the energy spectra of the scattered photons. (The terms“photon spectrum,” “energy spectrum,” and “photon energy spectrum” areused interchangeably throughout the present disclosure to refer to thephoton energy spectral distribution, or the number of counts detected ineach energy channel.) For example, region 1 may include the 511 keV lineproduced from positron production and annihilation in the target voxeland, region 2 may include an energy band beginning at 600 keV andextending toward some higher-energy limit programmed or entered into thedata reduction system. The higher energy region can be quite broad; inan exemplary embodiment it begins at 600 keV and extends upwards toabout 2 MeV. In an alternative embodiment, either or both regions may beas a narrow as a single detection channel if counting statistics areadequate. The exact energy limits will be determined by the specificrequirements of a particular application, and may depend upon thecounting statistics, detector resolution, or signal-to-noise ratiosobtained in the higher energy channels in a particular application. Itwill be understood that 600 keV is an arbitrary limit, and any energyrange may be used that excludes the 511 keV peak, although, as discussedabove, energies above 150 keV may be preferable. In practice, thehigher-energy region may be selected by choosing a center energy andthen selecting a width such that the statistical fluctuations over thatregion are no longer dominated by noise.

Let R(Z) represent the ratio of the scattered photon intensity in region1 to the scattered photon intensity in region 2. In FIG. 4, R(Z) isplotted for approximately equal effective masses of material with Z=92,82, 29, and ˜7 (uranium, lead, copper, and B₂O₃ surrounded by paper,respectively). In FIG. 4, E₁ was chosen so that the numerator of R(Z)was the integral under the 511 keV peak; E₂ was approximately 600 keV+/−5 keV.

As can be seen from FIG. 4, the dependence of R(Z) upon atomic number Zis quite pronounced. Generally, the higher the average Z in the target,the lower the value of R(Z). The ratio is largely independent of thetotal mass of material contained in the voxel. Thus, the average Z in avoxel may be determined by counting photons scattered in each of anappropriately selected energy region 1 and 2; computing the ratio R(Z),and comparing the result to an R(Z) curve such as that shown in FIG. 4.The known R(Z) curve used for this comparison may be determinedempirically by placing test targets of known Z in the path of the beam,and measuring R from several such test targets having a range of Z.Alternatively, an R(Z) curve could be determined by analytic orstatistical modeling of the interactions occurring in the target.

It should be noted that depending upon the size of the interval E₁ thatincludes the annihilation peak, a subtraction of the continuous portionof the photon energy spectrum in this energy interval may be desirablebefore computing the ratio R(Z). For example, if the detector has abroad energy resolution (relative to the width of the 511 keV peak), theinterval E₁ may include contributions from the continuous portion of thephoton energy spectrum. In order to get an accurate measure of thenormalized counts in the annihilation peak, these contributions may beestimated and subtracted out. In an exemplary embodiment, thecontributions of the continuous portion may be estimated by averagingthe continuous portion of the photon energy spectrum on either side ofthe energy interval E₁.

In some embodiments, the photon intensity in region 1 may be measured ina different scattering direction from the photon intensity in region 2.In such an embodiment, the photon intensity in one of the two regionsmay be corrected for the difference in measurement angle prior tocomputing the ratio. The correction can be determined empirically,analytically, or by statistical modeling to determine the angulardistribution of scattered photons and/or any differences in detectionefficiency between the detectors.

In another exemplary embodiment, the scattered photon energy spectrumfrom a voxel can be used in conjunction with the photons scattered fromneighboring regions of the target to indicate the presence of a high-Zmaterial, as follows. Presence of a signifycant quantity of high-Zmaterial in one voxel may result in strong absorption of the beam, whichin turn results in a reduced flux incident upon the voxels downstream inthe beam path. Thus, the energy spectra of photons scattered fromdownstream voxels may exhibit a reduction in signal strength. Similarly,if a high-Z material is present in between the voxel under interrogationand the detector (out of the path of the photon beam), then the photonsscattered from the voxel under interrogation may be attenuated byabsorption on their way to the detector, causing a reduction in signalfrom the interrogated voxels. In an exemplary embodiment, athreat-detection system may look for such spatial correlations in theattenuation of the signal and use them to determine or verify thepresence of a high-Z material both along the beam and/or along the pathfrom the voxel under interrogation to the detector. For example, whensuch a correlation is detected, it may alert an operator and/or triggerfurther investigation of the target or of a region of the target, suchas with NRF imaging.

Determination of the Mass:

The mass of the material contained in each voxel can be estimated by aniterative method using as input the number of photons measured in thecontinuous region of the photon energy spectrum above a given energy,such as 600 keV (see FIG. 3). The choice of energy region is arbitrary,and any region may be used that provides adequate counting statistics.The optimal energy regions for a particular application will depend uponthe design specifics of the scattering system. The energy regions usedin a particular application may be selected empirically by adjustingparameters until the greated sensitivity for mass determination isachieved. Alternatively, energy regions may be selected usingstatistical modeling methods to compute estimated optimum values.

In an exemplary embodiment of this method, the mass in the first voxelinterrogated by the photon beam is determined using the incident fluxF₁, and then the mass is used to estimate the flux incident upon thenext voxel in the beam path, and so on along the beam, as follows.Referring to FIG. 2, which schematically illustrates the detection ofscattered photons from several voxels along the beam path, the photoncount from the i'th voxel, S_(i), into direction θ with energies fallingin the range E_(γ), is given by:

S _(i)(E _(γ) ,θ,Z _(i))=G _(i)(E _(γ) ,θ,Z _(i))F ₁ M _(i)  Equation 1

Using Equation 1, the R(Z) ratio is given by:

R _(i)(θ,Z _(i))=S _(i)(E ₁ ,θ,Z _(i))/S _(i)(E ₂ ,θZ _(i))=G _(i)(E ₁,θ,Z _(i))/G _(i)(E ₂ ,θ,Z _(i))  Equation 2

where G_(i)(E_(γ), θ, Z_(i)) is a known factor dependent on the averageZ value (Z_(i)), the energy of the measured photons and the angle θ ofscattering from the i'th voxel, F₁ is the total photon flux entering thei'th voxel; and M_(i) is the mass contained in the i'th voxel. (If thevoxel under consideration is the very first target voxel struck by theincident beam, then F_(i) is the incident flux F₁ discussed above.) Asdiscussed above, E₁ preferentially includes the annihilation peak at 511keV, while E₂ is preferably some higher energy range. Also as discussedabove, depending upon the size of the interval E₁ that includes theannihilation peak, a subtraction of the continuous portion of the photonenergy spectrum in this energy interval may be desirable.

The factor G_(i)(E_(γ), θ, Z_(i)) may be determined either empiricallyor by analytical or statistical modeling. G_(i)(E_(γ), θ, Z_(i)) isreferred to herein as the scattering kernel. It represents the flux ornumber of photons scattered in the measurement direction θ, scatteredfrom a material having atomic number Z_(i) and mass normalized to 1gram, for a unit total incident flux per cm². The kernel depends uponthe parameters defining the shape of the flux incident upon the i'thvoxel. For example, for a bremsstrahlung source, the kernel depends uponthe endpoint energy of the electron beam used to generate thebremsstrahlung photons. Thus, the kernel must be determined for theparticular photon source used in any given embodiment.

Data tables or analytic expression of the kernel G_(i)(E_(γ), θ, Z_(i))may be provided to the processor, in data reduction software that usesEquation 1 to determine the mass M_(i) in the i'th voxel. For example,the kernel G_(i)(E_(γ), θ, Z_(i)) could be determined in advanceempirically by placing test targets of known Z_(i) and known M_(i) invoxel i, and measuring S_(i) using known incident photon flux F_(i) forseveral such test targets having a range of average Z_(i). Where suchtest masses are used, the calibration of the G measurement must takeinto account the size of the interrogated voxel. This can be achievedby, for example, using test targets of smaller cross-sectional area thanthe incident photon beam. Then the product of the areal density of thetest target and the cross-sectional area of the test target provides thetotal mass of the interrogated voxels for the calibration.Alternatively, instead of direct measurement, the kernel G_(i) could bedetermined by Monte Carlo modeling of the interactions occurring in thetarget and incorporating the detector and beam geometry. Or, the kernelG_(i) could be determined by analytic modeling either in advance or inreal time. Thus, exemplary values may be stored by the system in datatables or as analytic expressions. Regardless of how the factor G_(i) isdetermined and stored, its values will be referred to herein as“predetermined values.” Generally, the factor G_(i)(E_(γ), θ, Z_(i))incorporates the Z-dependence and spatial dependence of the scatteringcross-section, the energy distribution of the photons incident on thei'th voxel (which, when a bremsstrahlung source is used, is dependentupon the spectrum of the electron beam used to create the bremsstrahlungsource), as well as the detector geometry.

The average Z in each voxel Z₁ along the beam may be determined by, forexample, the ratio method discussed above. Using the average atomicnumber Z₁ in the first voxel to evaluate G₁, together with the measuredscattered photon count S₁ from the first voxel and the known initialphoton flux F₁, the average mass M₁ in the first voxel may be determinedfrom Equation 1.

The attenuation resulting from interaction of the incident photon beamwith the contents of the first voxel can be estimated from the extractedvalues of Z₁ and M₁, so that the flux F₂ incident on the second voxel(the next voxel along the beam) can be estimated. Using the averageatomic number Z₂ in the second voxel (determined, for example, using theratio method described above) to evaluate G₂, together with the measuredscattered photon count S₂ from the first voxel and the estimated photonflux F₂ incident on the second voxel, the mass M₂ in the second voxelmay be determined from Equation 1. The processor may be programmed torepeat this calculation for all voxels along the beam, estimating theflux F₁ incident on each voxel, and using the average Z value Z_(i) tocompute the mass M_(i) for each voxel.

In one embodiment, the values Z_(i), or M_(i), or both, are represented,for example, by colors or shading or points plotted on a visual displayto create an image of the mass- or Z-distribution along the beam. Byscanning the beam through the target and repeating this process, a 3-Dimage of the target's contents may be constructed.

In another exemplary embodiment, the exit flux from the container may bemeasured, to provide an additional constraint on the massdeterminations. The exit flux and its energy spectrum F_(E)(E) may bemeasured, similarly to the measurement of the initial flux F₁ describedabove, by placing a flux measurement devise in the beam path downstreamof the target, as illustrated schematically by reference number 74 inFIG. 2. Alternatively, the exit flux may be determined by other types oftransmission detection in the beam path, as illustrated schematically inFIG. 1. For example, transmission detector 24 may be an X-ray imagingdetector that can provide spatial information about the transmitted fluxas well as a measurement of the total transmitted flux. As anotherexample, the exit flux may be deduced from transmission NRF measurementsusing reference scatterers 28 and detector array 36. The exit fluxmeasurement can provide an additional constraint to the mass measurementas follows.

The step by step derivation of the mass in each successive voxelterminates at the end of the container with a predicted exit flux, P(E),where the explicit energy distribution of the flux as well as the totalflux may be predicted, based upon the energy distribution of theincident flux and of the measured scattered photons. This flux may becompared to the measured exit flux F_(E)(E) to correct the predictedmass measurement. To begin an exemplary process of correcting thepredicted mass distribution, the energy averaged flux difference,D(E)=F_(E)(E)−P(E), may be used to change the mass of each voxel alongthe path according to the influence on the total photon flux of theoriginal Z_(i) and M_(i). That is, the difference D is apportioned toeach voxel in proportion to the attenuation of that voxel to thetransmitted flux in the process of the derivation of Z_(i) and M_(i).Next, this first order mass distribution is modified in a minimizationprocedure so that the incident flux energy distribution and the exitflux energy distribution match as closely as possible. In addition, theenergy spectrum of the exit flux may be compared to that of thepredicted exit flux (P)E, and the measured average Z of each targetvoxel may similarly be adjusted in the minimization process, inparticular to take advantage of the large variation of theenergy-dependence of photon absorption with Z.

In another embodiment, aspects of the voxel geometry, such as diameteror orientation, may be varied to improve the comparison between thepredicted and measured exit flux energy spectrum and/or refine themeasurements of mass in each voxel. For example, a redefined voxelhaving twice the area, but one-half the thickness, of the original voxelmay contain the same mass, but yield a very different photonattenuation. (Mathematically, this is captured in the geometrydependence of G_(i), the scattering kernel.) Such voxel redefinition maybe achieved by, for example, changing the collimation of the photon beamand/or the detectors.

In an alternative embodiment, improved resolution in the average atomicnumber and mass estimates may be obtained by computing a correlationbetween R(Z) as defined above and the normalized intensity of theannihilation peak at 511 keV. This correlation may be obtained by, forexample, plotting R(Z) against the integrated counts in the 511 keVpeak. Because both of these variables have strong dependence upon Z, andthe peak intensity depends upon the mass in the interrogated voxel, thiscorrelation may provide improved separation between species of differentatomic number and/or different masses. For example, lighter species haverelatively large R(Z) (see FIG. 4) but relatively small counts in theannihilation peak, while heavier species have small R(Z) and largernumbers of counts in the annihilation peak. This correlation isillustrated in the plot In FIG. 5.

Each point in FIG. 5 represents a different mass and of the speciesidentified in the legend. The target masses used for the measurements inFIG. 5 are provided in Table 1 below. Each mass listed in the tablecorresponds to a data point in FIG. 5. The target mass can be matchedwith its corresponding data point by noting that the normalizedannihilation peak intensity (the x-axis value) increases monotonicallywith increasing mass for a given species. Thus, for example, the 0.54 kgSn target corresponds to the leftmost Sn point in FIG. 5 (lowestnormalized peak intensity), and the 1.61 kg Sn target corresponds to therightmost Sn point (highest normalized peak intensity). The y-axis,R(Z), of FIG. 5 was measured and computed using the number of counts inthe annihilation peak as the numerator, and the number of counts in anenergy interval of +/−5 keV around 1000 keV for the denominator. Thephoton source was a bremsstrahlung beam generated using a 2.8 MeVelectron beam.

TABLE 1 Masses of Targets Used to Generate Data in Figure 5 Species Mass(kilograms) B₂O₃ 0.85, 1.24 C 1.08 Al 0.70, 1.40, 2.10, 2.8 Fe 1.07,2.14, 3.2, 4.27 Sn 0.54, 1.08, 1.61 Pb 3.35, 6.69, 10.04 U 1.10

From the data in FIG. 5 it can be seen that a given measured value ofR(Z) may correspond to more than one average atomic number. This is sobecause of experimental uncertainty in the measured value of R(Z), andmay be particularly problematic for lower average atomic numbers, wherebackground effects such as ambient scatter can dominate the R(Z)measurement. As an example, from FIG. 5 it can be seen that a measuredR(Z) of approximately 1600 may be observed using samples of carbon,B₂O₃, or aluminum.

Plotting R(Z) against the normalized annihilation peak intensity,however, can provide improved separation, facilitating the distinctionsamong different species that may yield similar measured R(Z) values.This is so because for a given voxel geometry and a given average atomicnumber, there is a limit to how much mass can be present in a voxel.Thus, for a given value of R(Z) that could correspond to more than oneaverage atomic number, a high peak intensity suggests the presence ofdenser material, increasing the probability that the voxel has a higheraverage atomic number. As an example, from FIG. 5 it can be seen thatwhile certain samples of carbon, B₂O₃, or aluminum all yield measuredratios of approximately 1600, plotting these ratios against thenormalized annihilation peak intensity provides some separation amongthese species.

A threat-detection system may analyze such correlations between measuredR(Z) and measured annihilation peak intensity to distinguish amongmultiple possible average Z values. Alternatively, the system may, giventhe measured R(Z) and annihilation peak intensities, compute and outputprobabilities that the voxel contains certain mass/average Zcombinations (probability p_(A) that the voxel contains a mass M_(A) ofmaterial with average Z Z_(A), and/or probability p_(B) that the voxelcontains a mass M_(B) of material with average Z Z_(B)). For example,consider the data shown in FIG. 5 for sample of carbon and aluminum.Since certain samples of both of these species yield a measured R(Z) ofapproximately 1600, the R(Z) measurement alone cannot distinguishbetween them. However, if the normalized annihilation peak intensity isrelatively high (for example, about 450), the probability may be quitelow that a sufficient amount of the light element carbon may be presentin the voxel under interrogation to yield the measured peak intensity.In contrast, the probability may be considerably higher that asufficient amount of the relatively heavier element aluminum may bepresent. The system may, in some embodiments, employ threat-detectionheuristics that use such probabilities to determine whether to notify anoperator, sound an alarm, and/or trigger further scanning such as NRFimaging or other imaging.

It should be noted that the measured annihilation peak intensity mayneed to be normalized for one or more factors in constructing acorrelation such as that shown in FIG. 5. First, an overallnormalization for incident flux may be required. In some embodiments,this overall normalization may be based upon the brightness of theincident beam. For example, in the illustration in FIG. 5, the number ofcounts in the annihilation peak was normalized to the total charge onthe bremsstrahlung target used to generate the beam (live time×electronbeam current). Alternatively, where the incident flux upon each voxel isestimated as described above, the annihilation peak intensity may benormalized to that estimated incident flux. In addition, a second, moresophisticated normalization factor may be applied where necessary ordesired for refinement of the correlation. This additional normalizationtakes into account the effects of the geometry of the voxel, asdescribed above in connection with determining the scattering kernel G.

Limits of the Mass of High-Z Material:

A voxel may contain a mixture of materials with the measured values of Zand M resulting from the summation of the elemental scattered photondistributions. (For the purposes of this discussion, the measured valuesof Z and M in a given voxel will be denoted by Z_(m) and M_(m)respectively.) For example, for two materials, one will haveZ_(H)>Z_(m l , and mass M) _(H), and the other Z_(L)<Z_(m), and massM_(L). In this case, the following relation holds for a given voxel;

M _(m) =M _(H) +M _(L)  Equation 3

For each energy region of the scattered photon energy spectrum, thefollowing relationship holds:

G(E _(γ) ,θ,Z _(m))M _(m) =G(E _(γ) ,θ,Z _(H))M _(H) +G(E _(γ) ,θ,Z_(L))M _(L)  Equation 4

Where flux, F, incident on the voxel, divides out. Eq. 4 can be used toset a limit on the mass M_(H) by setting Z_(L) to the lowest value forwhich G(E_(γ), θ, Z_(L)) has been measured. Using Equation 3 andEquation 4:

M _(H) =M _(m) [G(E _(γ) ,θ,Z _(m))−G(E _(γ) ,θ,Z _(L))]/[G(E _(γ) ,θ,Z_(H))−G(E _(γ) ,θ,Z _(L))]  Equation 5

As can be seen from the exemplary data in FIG. 3, in the limit of Z_(L)small compared to Z_(m), G(E_(γ), θ, Z_(L)) is much less than G(E_(γ),θ, Z_(m)) Also, G(E_(γ), θ, Z_(L)) is much less than G(E_(γ), θ, Z_(H)).With these approximations, Equation 5 reduces to:

M _(H) =M _(m) [G(E _(γ) ,θ,Z _(m))/G(E _(γ) ,θ,Z _(H))]  Equation 6

As an example using the data from FIG. 3, with Z_(m)=29, Z_(H)=82,M_(H)˜ 1/10 M_(m).

The following example illustrates an alternative method of comparing alimit for the mass M_(H).

Defining G_(i)(0.51 MeV, θ, Z_(i))=G³(Z) (G of the annihilation peak),and G_(i)(0.6 MeV, θ, Z_(i))=G^(c)(Z) (G of the continuum energy chosentor the measurement, where 0.6 MeV can be replaced by whatever energy ischosen), and assuming an admixture of the two mass components M_(L) andM_(H), the measured R value is given by:

R(Z _(m))=[G ³(Z _(H) +G ³(Z _(L))M _(L) ]/[G ^(c)(Z _(H))M _(H) +G^(c)(Z _(L))M _(L)],  Equation 7

which can be rewritten as

R(Z _(m))=[G ^(a)(Z _(H))/(G ^(c)(Z _(H))][M _(H) +M _(L) G ^(a)(Z_(L))/G ^(a)(Z _(H))]/[M _(H) +M _(L) G ^(c)(Z _(L))/G ^(c)(Z_(H))]  Equation 8

This can be reduced to:

R(Z _(m))=R(Z _(H))[M _(H) +M _(L) G ^(a)(Z _(L))/G ^(a)(Z _(H))]/[M_(H) +M _(L) G ^(c)(Z _(L))/G ^(c)(Z _(H))]  Equation 9

Equation 9 can be used to estimate an upper limit the contribution of ahigh-Z material that is embedded into a lower Z matrix. As an example,suppose Z_(m)=29 for a voxel of mass=M_(m) (where Z_(m) and M_(m) may bemeasured using the methods described above). If the high Z material isassumed to be uranium, an upper limit of the contribution of uranium(Z_(H)=92) is obtained by extrapolating Z_(L)=1. In this example, theterm G^(a)(Z_(L))/G^(a)(Z_(H)) is taken to be 1 as the data in FIG. 3indicates, and G^(c)(Z_(L))/G^(c)(Z_(H)) is or order 1/100), yieldingM(Z_(H)=92)< 1/7 M_(m). The simplicity of this results depends on therapid variation of the continuum flux with Z and the relative constancyof the annihilation yield for roughly equal mass targets.

What is claimed is:
 1. A method for analyzing material in a voxel of atarget, the method comprising: (a) illuminating the voxel of the targetwith a photon beam; (b) measuring with at least one photon deteetor afirst number of photons scattered from the voxel in a firstpredetermined energy range and in a first predetermined measurementdirection; (c) measuring with at least one photon detector a secondnumber of photons scattered from the voxel in a second predeterminedenergy range and in a second predetermined measurement direction; (d)determining in a processor a ratio of the first number of photons to thesecond number of photons; (e) determining in a processor an averageatomic number of the material in the voxel using the ratio: (f) choosingat least one predetermined average atomic number range; (g) based uponthe average atomic number determined not being within any chosenpredetermined average atomic number range, taking no further action withrespect to the voxel; aND (h) based upon the average atomic numberdetermined being within a chosen predetermined average atomic numberrange, taking an action with respect to the voxel chosen from the groupconsisting of: scanning the voxel at a higher resolution; performing aNRF scan of the voxel; scanning the voxel with another scanning method;displaying in an output device an image of at least a part of the targetto an operator; and notifying the operator by means of an output deviceof a potential threat.
 2. The method of claim 60, wherein the firstpredetermined measurement direction equals the second predeterminedmeasurement direction.
 3. The method of claim 60, wherein the firstpredetermined energy range equals the second predetermined energy range.4. The method of claim 60, wherein the first predetermined energy rangeincludes 511 keV.
 5. The method of claim 63, wherein the secondpredetermined energy range excludes 511 keV.
 6. A method for analyzingmaterial in a voxel of a target, the method comprising: (a) illuminatingthe voxel of the target with a photon beam; (b) measuring with at leastone photon detector a first number of photons scattered from the voxelin a first predetermined energy range and in a first predeterminedmeasurement direction; (c) measuring with at least one photon detector asecond number of photons scattered from the voxel in a secondpredetermined energy range and in a second predetermined measurementdirection; (d) determining in a processor a ratio of the first number ofphotons to the second number of photons; (e) determining in a processoran average atomic number of the material in the voxel using the ratio;(f) based upon the average atomic number determined not exceeding apredetermined average atomic number, taking no further action withrespect to the voxel; and (g) based upon the average atomic numberdetermined exceeding a predetermined average atomic number, taking anaction with respect to the voxel chosen from the group consisting of:scanning the voxel at a higher resolution; performing a NRF scan of thevoxel; scanning the voxel with another scanning method; displaying in anoutput device an image of at least a part of the target to an operator;and notifying the operator by means of an output device of a potentialthreat.
 7. The method of claim 65, wherein the first predeterminedmeasurement direction equals the second predetermined measurementdirection.
 8. The method of claim 65, wherein the first predeterminedenergy range equals the second predetermined energy range.
 9. The methodof claim 65, wherein the first predetermined energy range includes 511keV.
 10. The method of claim 68, wherein the second predetermined energyrange excludes 511 keV.
 11. A method of scanning a target for potentialthreats, the method comprising: (a) for each of a plurality of voxels inthe target: (i) illuminating the voxel with a photon beam; (ii)measuring with at least one photon detector a first number of photonsscattered from the voxel in a first predetermined energy range and in afirst predetermined measurement direction; (iii) measuring with at leastone photon detector a second number of photons scattered from the voxelin a second predetermined energy range and in a second predeterminedmeasurement direction; (iv) determining in a processor a ratio of thefirst number of photons to the second number of photons; (v) determiningin a processor an average atomic number of material in the voxel usingthe ratio; (b) choosing at least one predetermined average atomic numberrange; (c) based upon the average atomic numbers determined not beingwithin any chosen predetermined average atomic number range, taking nofurther action with respect to the target; and (d) based upon apredetermined number of the average atomic numbers determined beingwithin a chosen predetermined average atomic number range, taking anaction with respect to the target chosen from the group consisting of:scanning at least one of the plurality of voxels at a higher resolution;performing a NRF scan of at least one of the plurality of voxels;scanning at least one of the plurality of voxels with another scanningmethod; displaying in an output device an image of at least a part ofthe target to an operator; and notifying the operator by means of anoutput device of a potential threat.
 12. The method of claim 70, whereinthe first predetermined measurement direction equals the secondpredetermined measurement direction.
 13. The method of claim 70, whereinthe first predetermined energy range equals the second predeterminedenergy range.
 14. The method of claim 70, wherein the firstpredetermined energy range includes 511 keV.
 15. The method of claim 73,wherein the second predetermined energy range excludes 511 keV.
 16. Amethod of scanning a target for potential threats, the methodcomprising: (a) for each of a plurality of voxels in the target; (i)illuminating the voxel with a photon beam; (ii) measuring with at leastone photon detector a first number of photons scattered from the voxelin a first predetermined energy range and in a first predeterminedmeasurement direction; (iii) measuring with at least one photon detectora second number of photons scattered from the voxel in a secondpredetermined energy range and in a second predetermined measurementdirection; (iv) determining in a processor a ratio of the first numberof photons to the second number of photons; (v) determining in aprocessor an average atomic number of material in the voxel using theratio; (b) based upon the average atomic numbers determined notexceeding a predetermined average atomic number, taking no furtheraction with respect to the target; and (c) based upon a predeterminednumber of average atomic numbers determined exceeding a predeterminedaverage atomic number, taking an action with respect to the targetchosen from the group consisting of: scanning at least one of theplurality of voxels at a higher resolution; performing a NRF scan of atleast one of the plurality of voxels; scanning at least one of theplurality of voxels with another scanning method; displaying in anoutput device an image of at least a part of the target to an operator;and notifying the operator by means of an output device of a potentialthreat.
 17. The method of claim 75, wherein the first predeterminedmeasurement direction equals the second predetermined measurementdirection.
 18. The method of claim 75, wherein the first predeterminedenergy range equals the second predetermined energy range.
 19. Themethod of claim 75, wherein the first predetermined energy rangeincludes 511 keV.
 20. The method of claim 78, wherein the secondpredetermined energy range excludes 511 keV.
 21. A system for analyzingmaterial in a voxel of a target, the method comprising: (a) a device forilluminating the voxel of the target with a photon beam; (b) at leastone photon detector configured to measure a first number of photonsscattered from the voxel in a first predetermined energy range and in afirst predetermined measurement direction; (c) at least one photondetector configured to measure a second number of photons scattered fromthe voxel in a second predetermined energy range and in a secondpredetermined measurement direction; (d) a processor configured: (i) todetermine a ratio of the first number of photons to the second number ofphotons; (ii) to determine an average atomic number of the material inthe voxel using the ratio; (iii) based upon the average atomic numberdetermined not being within a chosen predetermined average atomic numberrange, to determine to take no further action with respect to the voxel;and (iv) based upon the average atomic number determined being withinthe chosen predetermined average atomic number range, to determine totake an action with respect to the voxel chosen from the groupconsisting of: scanning the voxel at a higher resolution; performing aNRF scan of the voxel; scanning the voxel with another scanning method;instructing an output device to display an image of at least a part ofthe target to an operator; and instructing the output device to notifythe operator of a potential threat; and (e) an output device configuredto: (i) display an image of at least a part of the target to anoperator; and (ii) notify the operator of a potential threat.
 22. Thesystem of claim 80, wherein the first predetermined measurementdirection equals the second predetermined measurement direction.
 23. Thesystem of claim 80, wherein the first predetermined energy range equalsthe second predetermined energy range.
 24. The system of claim 80,wherein the first predetermined energy range includes 511 keV.
 25. Thesystem of claim 83, wherein the second predetermined energy rangeexcludes 511 keV.
 26. A system for analyzing material in a voxel of atarget, the method comprising: (a) a device for illuminating the voxelof the target with a photon beam; (b) at least one photon detectorconfigured to measure a first number of photons scattered from the voxelin a first predetermined energy range and in a first predeterminedmeasurement direction; (c) at least one photon detector configured tomeasure a second number of photons scattered from the voxel in a secondpredetermined energy range and in a second predetermined measurementdirection; (d) a processor configured: (i) to determine a ratio of thefirst number of photons to the second number of photons; (ii) todetermine an average atomic number of the material in the voxel usingthe ratio; (iii) based upon the average atomic number determined notexceeding a predetermined average atomic number, to determine to take nofurther action with respect to the voxel; and (iv) based upon theaverage atomic number determined exceeding a predetermined averageatomic number, to determine to take an action with respect to the voxelchosen from the group consisting of: scanning the voxel at a higherresolution; performing a NRF scan of the voxel; scanning the voxel withanother scanning method; instructing an output device to display animage of at least a part of the target to an operator; and instructingthe output device to notify the operator of a potential threat; and (e)an output device configured to: (i) display an image of at least a partof the target to an operator; and (ii) notify the operator of apotential threat.
 27. The system of claim 85, wherein the firstpredetermined measurement direction equals the second predeterminedmeasurement direction.
 28. The system of claim 85, wherein the firstpredetermined energy range equals the second predetermined energy range.29. The system of claim 85, wherein the first predetermined energy rangeincludes 511 keV.
 30. The system of claim 88, wherein the secondpredetermined energy range excludes 511 keV.
 31. A system for scanning atarget for potential threats, the method comprising: (a) a device forilluminating a plurality of voxels in the target with a photon beam; (b)at least one photon detector configured to measure a first number ofphotons scattered from each of the plurality of voxels in a firstpredetermined energy range and in a first predetermined measurementdirection; (c) at least one photon detector configured to measure asecond number of photons scattered from each of the plurality of voxelsin a second predetermined energy range and in a second predeterminedmeasurement direction; (d) a processor configured: (i) to determine foreach of the plurality of voxels a ratio of the first number of photonsto the second number of photons; (ii) to determine an average atomicnumber of the material in each of the plurality of voxels using theratio; (iii) based upon the average atomic numbers determined not beingwithin a predetermined average atomic number range, to determine to takeno further action with respect to the target; and (iv) based upon apredetermined number of the average atomic numbers determined beingwithin the predetermined average atomic number range, to determine totake an action with respect to the target chosen from the groupconsisting of: scanning at least one of the plurality of voxels at ahigher resolution; performing a NRF scan of at least one of theplurality of voxels; scanning at least one of the plurality of voxelswith another scanning method; instructing an output device to display animage of at least a part of the target to an operator; and instructingthe output device to notify the operator of a potential threat; and (e)an output device configured to: (i) display an image of at least a partof the target to an operator; and (ii) notify the operator of apotential threat.
 32. The system of claim 90, wherein the firstpredetermined measurement direction equals the second predeterminedmeasurement direction.
 33. The system of claim 90, wherein the firstpredetermined energy range equals the second predetermined energy range.34. The system of claim 90, wherein the first predetermined energy rangeincludes 511 keV.
 35. The system of claim 93, wherein the secondpredetermined energy range excludes 511 keV.
 36. A system of scanning atarget for potential threats, the method comprising: (a) a device forilluminating a plurality of voxels in the target with a photon beam; (b)at least one photon detector configured to measure a first number ofphotons scattered from each of the plurality of voxels in a firstpredetermined energy range and in a first predetermined measurementdirection; (c) at least one photon detector configured to measure asecond number of photons scattered from each of the plurality of voxelsin a second predetermined energy range and in a second predeterminedmeasurement direction; (d) a processor configured: (i) to determine foreach of the plurality of voxels a ratio of the first number of photonsto the second number of photons; (ii) to determine an average atomicnumber of the material in each of the plurality of voxels using theratio; (iii) based upon the average atomic numbers determined notexceeding a predetermined average atomic number, to determine to take nofurther action with respect to the target; and (iv) based upon apredetermined number of the average atomic numbers determined exceedinga predetermined average atomic number, to determine to take an actionwith respect to the target chosen from the group consisting of: scanningat least one of the plurality of voxels at a higher resolution;performing a NRF scan of at least one of the plurality of voxels;scanning at least one of the plurality of voxels with another scanningmethod; instructing an output device to display an image of at least apart of the target to an operator; and instructing the output device tonotify the operator of a potential threat; and (e) an output deviceconfigured to: (i) display an image of at least a part of the target toan operator; and (ii) notify the operator of a potential threat.
 37. Thesystem of claim 95, wherein the first predetermined measurementdirection equals the second predetermined measurement direction.
 38. Thesystem of claim 95, wherein the first predetermined energy range equalsthe second predetermined energy range.
 39. The system of claim 95,wherein the first predetermined energy range includes 511 keV.
 40. Thesystem of claim 98, wherein the second predetermined energy rangeexcludes 511 keV.